This invention relates to an air conditioning system for automotive vehicles, and more particularly to an overheat detecting device which is provided in an air conditioning system of this kind and adapted to detect abnormal overheating of refrigerant, as caused by a shortage of same in the refrigerating cycle.
Conventionally, an air conditioning system for automotive vehicles, such as a refrigerating system, is provided with an automatic expansion valve so as to control overheating of refrigerant in the air conditioning system. However, the automatic expansion valve has the drawback that, when refrigerant in the refrigerating cycle is leaked below a predetermined amount, the refrigerant in the inlet pipe line to the compressor becomes overheated to such a high level that the automatic expansion valve cannot properly operate, i.e. superheating of refrigerant, resulting in overheating of the compressor. On such occasion, while the temperature of refrigerant in the inlet pipe line to the compressor is abnormally increased, the pressure of the refrigerant is reduced. It is, therefore, possible to detect the overheating of refrigerant by comparing the temperature of the refrigerant and the pressure of same. To detect the temperature and pressure of the refrigerant is known e.g. from U.S. Pat. Nos. 3,047,696 and 3,686,892 wherein are provided overheat detecting devices which are adapted to stop operation of the compressor or to give warning in response to a sensor output signal indicative of detected overheating of refrigerant.
However, according to conventional overheat detecting devices of this kind inclusive of the above proposed ones, a switch means for detecting the overheating of refrigerant is arranged in the suction chamber of the compressor or in the inlet pipe line to the compressor while the compressor is usually installed in the engine room. Accordingly, the switch means is subject to direct heat radiation from the engine. Such conventional overheat detecting device in general comprises a diaphragm which includes two thin plates made of metal and joined together at their peripheral edges, a thermo-sensitive means communicating with the interior of the diaphragm and inserted in a detecting space for detecting of the temperature of refrigerant, such as in the suction chamber of the compressor or the inlet pipe line thereto, and a diaphragm chamber accommodating the diaphragm and communicating with the detecting space. The interior of the diaphragm and the interior of the thermo-sensitive means form an enclosed space in which thermal expansion medium is filled, and the thermal expansion medium has its pressure varying with a change in the temperature of the detecting space. The diaphragm is displaced in response to the difference between the pressure of the thermal expansion medium within the enclosed space and the pressure within the diaphragm chamber, so that it can function as a switch means which controls on-off operation of the driving circuit for the compressor or a warning device. If the overheat detecting device is arranged in a place where it can be easily heated, such as in the suction chamber of the compressor or in the inlet pipe line to the compressor, when the air conditioning system is at rest, the ambient temperature of the overheat detecting device and the temperature of the thermal expansion medium within the enclosed space become the same and sometimes rise up to about 90.degree. C., due to little refrigerant circulation through the detecting space. Further, the pressure in the diaphragm chamber is decreased to a relatively low level since the refrigerant in the diaphragm chamber is drawn into the refrigerating cycle by the compressor. Thus, the difference between the pressure in the diaphragm chamber and the pressure in the enclosed space frequently becomes larger than that where the switch means of the overheat detecting device is closed or turns on as a result of abnormal overheating of the refrigerant. As a consequence, a large stress corresponding to the difference between the pressure in the diaphragm chamber and the pressure in the enclosed space is produced in the diaphragm to displace same beyond its limit of elasticity, sometimes causing permanent deformation of the diaphragm. Further, repeated operation of the refrigerating system can further increase the degree of permanent deformation of the diaphragm, resulting in malfunction of the overheat detecting device.